Piezoelectric means for converting pressure variations into potential variations



VARIATIONS 49 W. KISTLER PIEZOELECTRIC MEANS FOR CONVERTING PRESSURE 0 POTENTIAL July 18. 19

VARIATIONS INT Filfid [-76, 1(priararf) FIG. 4

May 16, 1950 Patented May 16, 1950 PIEZOELECTRIC MEANS FOR CONVERTING PRESSURE VARIATIONS INTO POTENTIAL VARIATIONS Walter Kistler, Wintcrthur, Switzerland, assigncr to Schweizerlsche Lokomotivund Maschlncnfabrik, Winterthur, Switzerland Application July 18, 1949, Serial No. 105,332 In Switzerland August 6, 1948 7 Claims. 1

My present invention relates to improvements in piezoelectric means for converting pressure variations into potential variations, as applied in particular for measuring the pressure variations in the cylinders of internal combustion engines. In such piezoelectric means the quartz system, which is prestressed in the direction oi. pressure, is arranged in form of a rod and incorporated in a metallic sleeve which is closed on the side of the pressure chamber.

In accordance with comparatively recent fundamental investigations, it is possible to construct piezoelectric units of particularly high accuracy by very rigidly bracing the pressure-responsive quartz elements, and by resiliently prestressing the said elements in the direction of the pressure.

Quartz units built on the basis of the abovecited conditions and which comprise two bored centrally prestressed quartz plates, however, have a disadvantage in that they are not reliable in operation, since they cannot be sufliciently sealed against penetration of combustion gases and moisture.

In order to obviate such disadvantage, quartz units are known in the art in which the quartz plates are accommodated in a metallic sleeve which is closed on the side facing the pressure chamber, such sleeve at the same time acting to elastically prestress the quartz plates. The resilience of this wide and relatively short sleeve, however, is too small, from which reason piezoelectric means oi such latter type are temperature-sensitive and only slightly pressure-responsive. It is conceded, however, that such latter type piezoelectric means has an advantage in that an axial bore may be used for delivering the electrical charge.

It is known on the other hand that quartz elements in form of rods are more highly pressureresponsive than elements in form of plates. In piezoelectric units provided with such quartz rod, the latter is pressed in direction oi! the normal crystal axis, and the arising electrical charge is taken oil the two sides of the rod, as schematically shown in Fig. 1. The correct bracing and prestressing of such quartzes, however, presents even greater diiflculties, especially as lateral bores have to be provided in the casing for carrying oil the electrical charge.

The said disadvantages are eliminated in accordanee with my present invention, in that at least two juxtaposed quartz rods are provided in the piezoelectric unit, which rods together form a cylinder. The said rods are so cut from the crystal that their neutral axes run parallel to 2 the pressure axis and their electrical axes are poled against each other and stand at right angles to the direction or pressure. The quartz rods are prestressed in the direction or pressure. and accommodated in a metallic sleeve which is closed on the side of the pressure chamber.

One form of my present invention is shown in the accompanying drawing, in which- Fig. 1 a perspective view of a prior-art quartz unit in form of a solid rod as cited above,

Fig. 2 a perspective view 01' a quartz unit according to my present invention, comprising a pair of juxtaposed semi-cylindrical quartz rods,

Fig. 3 a longitudinal section through the piezoelectric means, and

Ii tig. 4 a modified sleeve mount for the quartz un As shown in Fig. 2, the pressure-responsive unit comprises two juxtaposed semi-cylindrical quartz rods l and 2 which together form 9. cylinder. The said rods are so cut from the quartz crystal that their neutral axes 3 run parallel to the pressure or cylinder axis and their electrical axes 4 have opposite poles and stand at right angles to the plane of separation. In this manner only positive charges, e. g., are set up on the curved surfaces or the two rods, and only negative charges are set up on the plane adjacent faces, as soon as a pressure is exerted on the rods.

As shown in Fig. 3, the two prestressed quartz rods l and 2 are suitably accommodated in a thin-walled sleeve 5 and abut against the steel cylinders 6 which are iorce fits in the said sleeve. The quartz rods are cut from the crystal so that their neutral axes run parallel to the pressure axis and their electrical axes have opposite poles and stand at right angles to the plane of separation.

The upper, ccnically tapered end of the sleeve 5 is fitted in the respective conical bore of the casing l. The lower end 511 of the sleeve, which is enlarged mushroom-like and which has to take up the gas pressure, is formed on its border with a thin diaphragm 8. The peripheral bead of the latter is rolled gastight in the foot or the casing I.

The charge produced on the curved surface of the quartz cylinder when stressing the latter in the longitudinal direction, which charge e. g. may be positive, is earthed through the sleeve. The charge arising in the interior of the cylinder at the plane of separation, which charge e. g. may be negative, is taken up through a thin copper strip 9 and fed through the conductor It to the contact I l which is lodged in the amber insulator supplied to the piezoelectric means through the pipes It. The drying substance it serves for abstracting any moisture which may have been introduced. The said substance is so disposed in the piezoelectric means that moisture which may have penetrated from the coolant chamber through the sealing i'aces into the quartz chamber, is adsorbed.

When pressure arises on the diaphragm 8, it is transmitted onto the quartz rods as well as onto the sleeve 5. The said rods are slightly compressed in accordance with their modulus or elasticity. The thin-walled sleeve I which is pre-' stressed in the longitudinal direction, will be relieved or distended in thesame measu e and elastically shortened. If the fleeve port on is semi-cylindrical quartz rods which are so cut from the crystal that their neutral axes run parallel to the pressure axis and their electrical had a substantial wall thickness, it would take up practically all of the pressure exerted on the diaphragm I, and only a very small portion of the pressure would be transmittedonto the quartz cylinder. The piezoelectric means, therefore, would not readily respond to pressure fluctuations and, thus, would be more sensitive to temperature. Since, however, the wall 5a of sleeve 5 is of the pressure exerted on the diaphragm 8. By reason of the prestress applied to sleeve 5, substantial tensile stresses and elongations are set up. in the portion Ba oi the sleeve.

- In the modified sleeve structure I! shown in Fig. 4, the sleeve portion Ila surrounding the quartz cylinder is not formed as cylindrical thin wall, but is corrugated. Such construct on gives the sleeve portion lia a higher elasticity in the longitudinal direction, whereby its wall l5a may be made thicker than in the iirst case. What I claim as new and desire to secure by Letters Patent is:

1. In a piezoelectric means for converting pressure variations, in particular in the cylinders of internal combustion engines, into potential variatons, in which the quartz unit is prestressed in the direction of pressure and is formed as rod and incorporated in a metallic sleeve which is closed on the side or the pressure chamber, a

quartz unit comprising at least two juxtaposed extremely thin, it absorbs only a few percent axes have opposite poles and stand at right angles to the direction of pressure.

2. In a piezoelectric means as set out in claim 1, an electrically conductive thin strip inserted in the surface of separation of the quartz rods, the said strip being adapted to take up the electrical charge arising on application of pressure and to deliver the same to an electric conductor.

3. In a piezoelectric means as set out in claim 1, including ducts for coolant water a quantity of drying substance lodged intermediate the coolant chamber and the quartz rods so that any leakage of traces of coolant first reaches the drying substance before passing to the quartz rods.

4. In a piezoelectric means as set out in claim 1, a 'quartz unit in which the sleeve portion surrounding the quartz rods is formed axially yieldable so as to be capable of elastically yielding to the pressure acting from the pressure chamber onto the sleeve end. i

5. In a piezoelectric means as set out in claim 1, a quartz unit in which the sleeve portion surrounding the quartz rods is corrugated to attain the desired resilience.

6. In a piezoelectric means as set out in claim '4, an element for axially prestresslng the quartz rods and comprising at least one axially bored cylinder adapted to be pressed into the open sleeve end so as to hold the quartz rods rigidly in the sleeve and, on the other hand to prestress the elastic sleeve portion. 7. In a piezoelectric means set out in claim 4, a sleeveoi which the portion surrounding the quartz rods is made extremely thin for the purpose oi attaining the desired resilience.

' WALTER, KISTLER.

REFERENCES CITED UNITED STATES PATENTS Name Date Hund June 7, 1932 Number 

